A thread, by definition is a light weight process. They are used to increase functionality and performance by performing multiple tasks at the same time, i.e. concurrently. There are two methods for implementing threads in Java,

Implementing an interface

Extending a class

I'd assume that the reader is familiar with the basic Object Oriented Paradigm concepts and understands terms like 'extending', 'interface' and 'class'. Now you may start wondering why are there two ways to create threads. This is because if a class is already an inherited class of some class other than 'Thread', then it cannot extend 'Thread' because multiple inheritance is not allowed in the Java programming language. So, in such cases we use 'Runnable' interface instead.

Now, let's jump onto the coding part on how to actually create threads. The first method is to extend or inherit the 'Thread' class. The 'Thread' class is defined in the package java.lang, which needs to be imported. Take a look at the code below to get a better idea,

Notice that both the methods make use of the 'run()' function, which is responsible for the actions of the thread. The 'Runnable' interface actually is nothing but a class containing only one abstract method, 'public abstract void run();'. Remember an interface only provides a design framework upon which classes can be implemented. It is also interesting to note that actually the 'Thread' class also implements the 'Runnable' interface.

It is better in the long term to use the implements runnable interface as this will make your code more extendible in the future as you can also have threads that subclass other classes other than just Thread. What is not mentioned is that the thread has a shared state from the calling class which brings in the Synchronized keyword when changing the state.